This invention relates to an optical fiber monitoring system which can be used to protect wires, cables, and the like by sensing overtemperature and/or mechanical stress conditions of such wires, cables and the like.
It is known to use an optic fiber monitoring system with a transformer or generator to monitor the operating temperature thereof and using a light source, which can be for example, a light emitting diode, which transmits light through an optical fiber conductor which passes through the transformer or a generator enclosure to impinge on a phototransducer. The electrical output from the phototransducer can be passed through an amplifier to a processor and then used to initiate an alarm if the temperature in the enclosure is beyond prescribed levels. The optical fiber conductor comprises an optical fiber core with one or more cladding material layers surrounding the core, and a jacket surrounding the core and the cladding material. The core exhibits an evanescent field which extends through the cladding material into the jacket at a selected range of optical wavelengths. The cladding material having a thickness whereby light attenuation in the conductor below a first temperature range is less than light attenuation in the conductor above the first temperature range.
It is also known to use a temperature monitoring arrangement comprising a light source situated at one end of an optical fiber section, and a light detector situated at the opposite end of the optical fiber section. The optical fiber section passes through an area in which the temperature is to be monitored, for example, a generator, transformer, or the like. The type of optical fiber section used can comprise a core and a cladding extending concentrically around the core, with the core and cladding being constructed of material such that the difference between their respective indices of refraction varies with temperature, at least over a predetermined temperature range, and, consequently, the amount of light passing through the fiber varies with changes in the temperature so long as the index of refraction of the core is greater than that of the cladding. It is also known to use an optical fiber comprising a flexible curved section constructed such that the amount of light passing through the fiber along the flexible curve section varies with changes in the radius of curvature of the curved section. In this arrangement, the sensor also includes means for varying the radius of curvature in response to changes in the temperature being monitored or other such condition.
Also known is the system which recognizes the problem of the impossibility of distinguishing the change in light transmission along an optical fiber caused by temperature change from other attenuations which may occur if, for example, the fiber is bent. In this system, the apparatus comprises means for directing light having at least two different wavelengths through an optical fiber section to a sensor element composed of material which changes color in dependence on changes in temperature and means for detecting the reflected light therefrom after it is passed through an optical fiber and for generating signals from the reflected light, thus determining the temperature to which the sensor element is being subjected.
It is also known to use an apparatus comprising an optical fiber section through which a laser transmits coherent light, with means for applying light and/or pressure to the optical fiber section to change the characteristics of the coherent light as it is transmitted through the fiber section.
Also known is a system which discloses an optical fiber coupler having a core and a cladding, the respective indices of refraction of the core and cladding being substantially the same in value at a particular temperature but having different temperature sensitivities, and temperature control means disposed around the end of the optical fiber section where coupling is to take place for generating a variation in the difference in the indices of refraction of the core and cladding in the vicinity of this end by controlling the temperature of this end, the differences in the indices being substantially zero at this end and gradually increasing in a limited region of the guide away from the end, whereby coupling of light beams into and out of this end of the optical coupler is facilitated.
It is also known for publications to discuss in detail the mathematical basis for the measurement of temperature using optical fibers. Also known is a treatise which discusses the respective effects of transmitted power versus temperature, index of refraction versus temperature, and induced attenuation coefficients versus temperature for various fibers.
None of the aforementioned prior art references discloses an fiber optical monitoring system for electrical conductors or the like, such as is disclosed by the present invention which system comprises an electrical apparatus having a plurality of conductors, an optic fiber situated in such close proximity to the plurality of electrical conductors that the optical fiber is affected by temperature changes of the plurality of conductors and accordingly the amount of light passing through the optical fiber is altered, a light source at a first end of the optical fiber, means for detecting light and producing an electrical signal proportional to the detected light, being located at a second end of the optical fiber opposite the first end, and means responsive to the electrical signal to open the circuit the plurality of conductors are part of when the electrical signal reaches a preset level.